Murder is the most serious crime on the books. A conviction can lead to life in prison and sometimes even the death penalty.
In some cases, a person who commits a killing can get the charge reduced to manslaughter or even involuntary manslaughter. Both of these can still lead to prison, but usually a far shorter sentence than an actual murder conviction.
In other cases, a person who does a killing can “walk” (that is, not suffer any conviction and not be sentenced to any custody time). We call these instances in California law of excusable or justifiable homicide.